Monitors in Syria say several air strikes struck targets in northwestern and central parts of the country on February 28, but a truce between government troops and moderate opposition forces appeared to be broadly intact on its second day.

The British-based Syrian Observatory for Human Rights says its monitors reported air strikes by either Syrian or Russian warplanes at six towns in Aleppo Province and one village in the province of Hama.

The observatory’s director, Rami Abulrahman, said it was not clear whether the air strikes hit areas covered by the truce.

Russia's monitoring center in Syria registered what it said were nine cease-fire violations since on February 27, but it said the truce was holding "in general."

Meanwhile, Russian military officials said they contacted a U.S.-operated coordination center in Jordan's capital, Amman, on February 28 for clarification about what the Kremlin says were artillery attacks on Syrian territory from Turkey.

The truce does not include territory held by Islamic State militants or the Al-Qaeda linked Al-Nusra Front.

Together, those militant extremist groups control about half of the territory in Syria.

The Russian center in Syria says U.S. officials have sent a list of 69 armed opposition groups which have agreed to the terms of the cease-fire.

The Russian center also said it has received declarations of agreement with the truce from 17 armed groups that are members of Syria's "moderate" opposition.

Saudi Arabia's foreign minister, meanwhile, said it is raising concerns with international powers about what it says are violations of the conditions of the truce by Russian and Syrian government forces.

A spokesman for Syria’s main opposition umbrella group said on February 28 that the High Negotiations Committee has asked U.S. officials for information about how the monitoring of the truce works but has yet to receive an answer.

The spokesman,Monzer Makhous, said the opposition will continue to back the truce -- despite what he said were 15 violations by government forces and their allies on February 27.

United Nations monitors reported only isolated fire during the first day of the truce.

UN diplomats said they hope the truce will provide a window for humanitarian aid to reach Syrian civilians who have been trapped for months in areas that are besieged by Syrian government forces.

UN envoy Staffan de Mistura said, that if the truce holds, he plans to start a second round of peace talks on March 7.